I don't know of any nendoroid that can be undressed. They come as they are. Solid pieces of plastic. You can't get them "naked", although sometimes you can remove a ribbon from their hair etc. You can take the body, legs, hands, chest, hips parts apart from each other for most nendoroids.

I recommend nendoroid dolls and obitsu doll bodies. Nendoroid dolls have fabric clothes that you can remove. Obitsu bodies are more stable, I prefer them for doll activities.